The present invention relates to an efficient audio signal amplifier and, more specifically, to a Class D amplifier using a passive resistive-capacitive (RC) network. More specifically, the present invention relates to Class D amplifier device utilizing a passive RC network that reduces current consumption and increases efficiency. The use of selectable output stages also reduces current consumption.
Class D amplifiers are in widespread use for a variety of applications. In audio applications, for example, the Class D amplifier has become an attractive alternative for applications that require high efficiency and low distortion. U.S. Pat. No. 4,689,819 to Killion describes the use of a Class D amplifier in a hearing aid application. In hearing aid applications, a premium is placed on the ability of the device to operate as close as possible to distortion free while maintaining suitable levels of amplifier performance.
A problem associated with prior art Class D amplifier devices is the use of active feedback loops that utilize substantial amounts of power. FIG. 1 illustrates a prior art class D amplifier circuit, denoted as 10, wherein a traditional feedback loop is established between the output and the input using an error amplifier. Essentially, circuit 10 uses an output stage 12 with an H-bridge output stage delivering output 14 that, in turn, drives load 16. The output signal 14 is coupled to the feedback loop 16 comprising of feedback impedance 18 which forms an input to error amplifier 20. The use of error amplifier 20 in a class D amplifier circuit, such as circuit 10, is well known in the industry. The output of the error amplifier 20 is fed to a comparator circuit 22 which, in turn, is driven by the 391 wave generator 24 which provides a clock signal for operating the output stage 12, and specifically for controlling the duty cycle of the output stage 12. Thus, the input signal 26 and feedback signal 16 are summed and provided to the error amplifier 20 which controls the stability of the circuit 10 and amplitude of the output signal 14 as a function of the feedback impedance 18 over the input impedance 28.
In a Class D amplifier circuit, such as circuit 10, the input and feedback signals are normally summed in an amplifier, such as error amplifier 20. This increases power consumption since the amplifier 20 is an active device requiring current to operate. Moreover, since the output stage 12 is always operating, power consumption at the output can be significant even for small signals outside the audible range. Accordingly, a Class D amplifier device with reduced internal power consumption would provide numerous advantages.
The present invention provides a Class D amplifier device with reduced power consumption. Essentially, power consumption is minimized by the use of a passive RC network. The output of the RC network directly feeds a comparator resulting in increased efficiency for low power audio applications. The RC network is configured to perform several functions, including the low pass filtering of the feedback signal to remove high frequency/high amplitude clock signals. In addition, the RC network provides a DC path from its output to reduce duty cycle errors. Additionally, the input signal receives low pass filtering, creating a generally flat frequency response. Finally, the clock signal is filtered to generate a triangle wave of good linearity and appropriate amplitude.
According to another embodiment, the present invention provides a Class D amplifier device with selectable output stages having gate drives that activate, depending on the load impendance and signal level, and therefore optimize current consumption and efficiency. By separating the gate drives into upper and lower stages, only those that are enabled corresponding to a suitable signal level amplitude are active, limiting consumption by other gate drive circuits in the output stage.
A technical advantage of the present invention is reduced current consumption by the driving stage in a Class D amplifier circuit.
Another technical advantages of the present invention is increased efficiency by the use of a passive RC network to generate the appropriate drive signals to the output.